Reimagining the origin myth is a fine way to revive an ailing franchise, as Batman Begins and Casino Royale have shown. Perhaps oppressed by imagery of a teeth-sucking Anthony Hopkins in Ridley Scott's squelchily horrible film of Hannibal - how do you top that climactic meal? - Thomas Harris, too, goes back to a clean slate, to beginnings. Hannibal Rising promises to explain how a human being became Dr Lecter.

There is danger here, as there is not in the case of superheroes and iconic spies, for if a character such as Lecter is explained as himself a victim of some original trauma, he is no longer fascinating as an avatar of absolute evil. It was that fascination, felt by FBI agent Clarice Starling (along with the reader) in The Silence of the Lambs and Hannibal, that fuelled those novels. If you have to explain a villain, it is perhaps best to do it on the hoof, in evocative flashback, as with Keyser Soze's origin story in The Usual Suspects. Or don't do it at all, as Bram Stoker did not with Dracula (but Francis Ford Coppola felt obliged to do in his otherwise rather faithful adaptation). Spelling it all out in a prequel, however, does risk bathos.

Reader, it's all Hitler's fault. The gothic Lithuanian castle that is home to Count Lecter and his family, including little Hannibal, is caught up in the fighting between Germany and the USSR in the 1940s. Hannibal's parents and the staff are killed, and something monstrous happens to his little sister. After the war, he is found by his uncle, and taken to live in Paris. A mathematical and scientific prodigy in his teens, he goes to medical school, and sets about tracking down the men who wronged him. They will not, we guess, long survive.

Meanwhile we have discovered how Hannibal acquired his celebrated "memory palace" (a kindly Jewish tutor called Jakov, back at the ancestral seat), and there is an odd subplot of art-historical detection. Combined with the somewhat touristy grooves of Hannibal's movements in Paris (whose placenames there has apparently been no time to edit), this prompts the dismaying thought that, after the brilliantly taut and claustrophobic procedurals earlier in the series, Harris has finally morphed into an upmarket version of Dan Brown.

There is still vastly more class and body in Harris's prose than in Brown's, of course (there could hardly be less). A skein of through-composed imagery involving birds is subtly nagging, and there are interesting tangential formulations, as when it is said of Jakov that he "made no effort to hide his mind, or to show it off, but he never pointed it directly at anyone"; or when, before his first kill, Hannibal experiences "a sharpness of vision, with edges of refracted red like ice on a window or the edge of a lens". There is, too, a superbly uncanny image of Notre Dame cathedral as a huge spider, scuttling through the city at night. But though there are still individual sentences and paragraphs to recall Harris's past mastery, the extremely short chapters are telling. Mere sequence, as of a film composed entirely of brief scenes, has replaced rhythm and suspense.

Since the redoubtable Clarice has not been born, young Hannibal needs another female foil. One is furnished in the shape of his uncle's wife. Luckily, she is Japanese, and so references to calligraphy, flower-arranging and haiku can do service for Lady Murasaki's character. The two conduct a peculiarly unpersuasive courtly love affair, at the unsatisfying conclusion of which Hannibal becomes, we are perhaps meant to suspect, ronin, or a samurai without a master.

In the interim, it appears to have taken some time for the anti-hero to refine his wit. One of his early attempts at a jokey payoff has him saying, of a man he has drowned in a vat of embalming fluid: "He arrived at a solution." The line begs for the help of Roger Moore's raised eyebrow. By the novel's final tidying, happily, Hannibal has become the morbidly wisecracking murderer we all know and love, informing his last victim, a taxidermist: "I've come to collect a head."

What is missing, unfortunately, is what the dustjacket promised: an explanation of "the evolution of his evil". At the end Hannibal is still a righteous avenging fury - pursuing justice, if sadistically. How does he turn into the demonic cannibal serial killer we first met in Red Dragon? There are rather chaste references to his acquisition of a taste for human flesh, but readers will thrill to no equivalent of the scene in George Lucas's Revenge of the Sith, where the Darth Vader suit is bolted around Anakin's scorched body. Hannibal is still not really Hannibal. This raises the most terrifying possibility of all: that another prequel lies in wait.